1. Field of the Invention
The present invention relates to a stepping motor, and more particularly to a stepping motor which is used in high-temperature environments and/or frictional heat generating environments therefore requiring heat resistance performance.
2. Description of the Related Art
Stepping motors, which are conveniently and well controllable, are used in various audio visual and office automation equipments. Especially, permanent magnet (PM) stepping motors are extensively used also in various control equipments carried in a vehicle due to low cost. Among the control equipments, for example, a following-distance sensor, and a headlamp optical axis controller are known. In a following-distance sensor, in which an ultrasonic sound wave is projected onto a preceding vehicle, and a reflected wave from the preceding vehicle is received for measuring a distance from the preceding vehicle, a crankshaft is attached to a rotary shaft of a stepping motor so as to swing a sensor in a given range. And, in a headlamp optical axis controller, a headlamp optical axis is controlled in the horizontal and vertical directions by means of gears attached to rotary shafts of stepping motors. Stepping motors incorporated in the following-distance sensor and the headlamp optical axis controller are required to stably operate with specified characteristics for a long period of time under high-temperature environments.
FIG. 3 is an explanatory view of a headlamp optical axis controller for a vehicle. A headlamp 50 is adapted to swing vertically about a support pin 51 as a fulcrum by means of a first screw 52, and horizontally about the support pin 51 by means of a second screw 53. The first screw 52 is rotated by a first motor 54 for vertical control of an optical axis of the headlamp 50, while the second screw 53 is rotated by a second motor 55 for horizontal control thereof. The first and second motors 54 and 55 have, on their rotary shafts, respective pinion gears 56 and 57 which are engaged with respective crown gears 58 and 59 attached to the ends of the first and second screws 52 and 53.
The first and second motors 54 and 55 are PM stepping motors which are structured so as to include a plate spring or a coil spring for the purpose of applying preload. The plate spring tends to incur an instable spring constant and cause variation in torque characteristics thus failing to achieve a stable motor performance. For this reason, the coil spring is preferred in view of preventing variation in torque characteristics.
FIG. 4 is a side view (partly sectioned) of a conventional PM stepping motor which includes a coil spring for preloading purpose as described above (refer to, for example, Japanese Patent Application Laid-Open No. H10-290548).
Referring to FIG. 4, a stepping motor 60 comprises a stator assembly 61, and a rotor assembly 62 rotatably disposed inside the stator assembly 61. The stator assembly 61 is composed of a first stator unit 68 and a second stator unit 69. The first stator unit 68 includes a coil 70 wound around a bobbin 71, and stator yokes 72 and 73, and the second stator unit 69 includes a coil 74 wound around a bobbin 75, and stator yokes 76 and 77. The rotor assembly 62 includes a rotary shaft 63, a sleeve 64, and a magnet 65 fixedly attached to the outer circumference of the sleeve 64. The rotary shaft 63 is rotatably supported by a bearing 66 and a bearing 67. A preloading mechanism 78 is disposed between the sleeve 64 and the bearing 67. The preloading mechanism 78 includes a coil spring 81 adapted to generate preloading force, and a spring holder which is composed of an outer case 79 formed of polyacetal (POM) resin and shaped into a cup-like configuration, and an inner case 80 formed of POM resin, shaped into a cup-like configuration and nested in the outer case 79, and which houses the coil spring 81. The preloading mechanism 78 thus structured applies preload against the rotor sleeve 64, that is to say, the rotor assembly 62. The preloading mechanism 78 slides against the bearing 67 while rotating together with the rotor assembly 62. A plain washer 82 is disposed between the sleeve 64 and the bearing 66.
A problem is caused when the stepping motor 60 described above is incorporated in a device, for example, the aforementioned following distance sensor, or headlamp optical axis controller, which is used in high-temperature environments, and/or in which the stepping motor 60 rotates at a so high speed as to generate frictional heat. Specifically, the preloading mechanism 78 is subject to a high temperature coming from the environmental heat and/or the frictional heat due to the high-speed rotating preloading mechanism 78 sliding against the bearing 67. Therefore, the spring holder, which is composed of the outer case and inner cases 79 and 80 formed of POM resin having a deflection temperature of about 135 degrees C. under a load of 1.8 MPa, is thermally decomposed thus failing to maintain its original configuration. This leads to failing to apply an appropriate preload and consequently deteriorating the performance of the stepping motor 60. The frictional heat is generated when the stepping motor 60 is rotated at a high speed, and can lead to a substantially high temperature even when the stepping motor 60 is used in non-high-temperature environments.